A steel bolt is mainly employed as a bolt which is a typical fastening member. A titanium alloy bolt is employed when a weight reduction or higher strength is required. A titanium alloy bolt requires a higher level of art for its manufacture than a steel bolt does. Art for manufacturing a titanium alloy bolt is proposed in, for example, Japanese Patent No. 2,982,579. The titanium alloy bolt disclosed in Japanese Patent No. 2,982,579 is manufactured from a Ti (titanium)-6% Al (aluminum)-4% V (vanadium) alloy.
The Ti-6% Al-4% V alloy is an alpha-beta alloy which is manufactured by adding an alpha-stabilizing element and a beta-stabilizing element to titanium. The alpha-beta alloy is difficult to work on at room temperature because of its high deformation resistance and low stretch ability. Hot forging performed at a high temperature is, therefore, employed for shaping an alpha-beta alloy by forging, since holding it at a high temperature lowers its deformation resistance and makes it easier to stretch.
However, a product of hot forging at a high temperature is seriously affected by the thermal expansion of the alloy. As a result, the forged product is undesirably low in dimensional accuracy. It is necessary to design a product of hot forging with a sufficiently thick cutting allowance for making up its low dimensional accuracy and a waste of the material is, therefore, inevitable.
The hot forging of a titanium material forms scale and oxide layers on its surface as its heavy oxidation takes place at a high temperature. The necessity for the removal of the scale and oxide layers adds to the cost of bolt manufacture.
Moreover, hot forging requires heat energy for heating the material to a high temperature.
On the other hand, cold forging can make a product close to a final product in shape, since it does not require any heat energy, or cause any lowering in dimensional accuracy that would result from thermal expansion.
Accordingly, it has been necessary to explore a titanium material which can be substituted for the alpha-beta alloy and is suitable for cold forging, and there have been proposed pure titanium and a beta titanium alloy as titanium materials to which cold forging is applicable.
Pure titanium is, however, too low in strength as a material for bolts of which high strength is required. The beta titanium alloy contains a by far larger amount of expensive material than the alpha-beta titanium alloy does, and it has a high deformation resistance. The necessity for a large amount of expensive material results in an expensive bolt and its high deformation resistance shortens the life of a die assembly. For these reasons, neither pure titanium nor the beta titanium alloy can be considered as a suitable material for bolts.
Therefore, it has been necessary to develop a titanium alloy bolt which can be manufactured by cold forging as a substitute for a bolt of pure titanium or a beta titanium alloy.